Published:20 February 2023,
Published Online:21 September 2022,
Received:05 June 2022,
Accepted:29 July 2022
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Poly(ethylene-co-octene)/multi-walled carbon nanotubes/carbon fibre (POE/MWCNTs/CF, 90:5:5, W/W/W) composites were prepared by melt-mixing, and the composites were then foamed in a batch process using supercritical carbon dioxide foaming method. The cell structure was analyzed for the microcellular samples prepared at foaming temperatures of 55, 60 and 65 ℃, and its effect was emphasized on the compression properties of the microcellular samples and the piezoresistive response (sensitivity and linear response range) of the assembled sensors. It was demonstrated that the microcellular sample foamed at 55 ℃ exhibited a relatively uniform cell structure, a narrower cell diameter distribution (mainly in the range of 10‒30 μm), and moderately thick and highly continuous cell walls, which endowed the microcellular sample with higher resilience, compression strength, compression modulus and electrical conductivity. The sensor assembled with this microcellular disk had a wider linear response range (0‒30% compression strain) and higher sensitivity (strain factor of 1.67), which were analyzed based on the cell structure. The sensor exhibited faster piezoresistive response and recovery performance and good repeatability, and showed higher stability and durability in the 1000 cycles of cyclic compression/release test with 30% strain. Moreover, the sensor could monitor typical human motions, such as finger pressing, elbow bending, squatting, and foot stepping, which corresponded to a wider compressive strain range . The results demonstrate that the microcellular conductive composites with more uniform cell structure and moderately thick and highly continuous cell walls foamed by supercritical fluid foaming method have good sensing performance.
通过釜压发泡法制备乙烯-辛烯共聚物(POE)基微孔复合材料圆片;由其封装成的传感器具有较宽的线性响应范围和较高的灵敏度,能检测典型的人体运动.
柔性压力传感器因具有质量轻、延展性、可调检测范围等优点,越来越多地应用在智能穿戴、人体运动和健康检测等领域[
柔性压阻式压力传感器的传感基片通常由聚合物基导电复合材料制备. 其中聚合物基体一般为具有较高柔软性和伸长率的弹性体材料,如聚二甲基硅氧烷(PDMS)[
与TPU、PDMS和橡胶等弹性体相比,乙烯-辛烯共聚物(POE)具有抗冲性能优异、热稳定性好、易加工等优点,被用作增韧剂(如增韧聚丙烯)或改性剂,关于以POE为基体制备多孔材料用于传感方面的研究还少有报道. Xu等[
POE,牌号8150,辛烯含量为25 mol%,密度0.868 g/cm3,熔体指数0.5 g/10min (2.16 kg,190 ℃),美国陶氏化学有限公司;MWCNTs,牌号XFQ038,直径8~15 nm,南京先丰纳米材料科技公司;CF,直径7 μm,深圳碳稀技术有限公司;CO2,工业级,纯度99.5%,市售.
转矩流变仪,型号RTOI-55/20,广州普同实验分析仪器公司;模压机,型号CH-0203,东莞创 宏仪器设备有限公司;高压釜发泡装置,主要由高压计量泵(型号500D,美国ISCO公司)和高压釜(型号DDC-1000N,盛世致远(北京)科技有限公司)构成.
将POE、MWCNTs和CF真空干燥8 h后,按90:5:5的质量比进行预混,加入转矩流变仪中进行混炼,制备复合材料. 混炼温度、转子转速和混炼时间分别设置为150 ℃、60 r/min和15 min. 将制备的复合材料放入模具型腔,将模具置于模压机中,在170 ℃下加热15 min后,在12 MPa的压力下保持15 min,冷却、脱模制得尺寸为100 mm × 100 mm × 1 mm的片材. 从片材中裁出直径为15 mm的圆片.
将上述圆片置于高压釜发泡装置中,采用Sc-CO2为物理发泡剂,通过快速泄压方式制备发泡样品,发泡过程详见文献[
1.3.1 复合材料的微观结构和热性能
将制备的POE基复合材料片材置于液氮中浸泡4 h后脆断,对脆断面进行真空喷金,采用扫描电子显微镜(SEM;型号Phenom Pure,荷兰Phenom)观察复合材料中MWCNTs和CF的分散和分布.
从制备的复合材料片材中切出约5 mg的薄片,置于示差扫描量热仪(DSC 3;型号METTLER,瑞士)中,在氮气气氛下以10 ℃/min的升温速率从25 ℃升温至200 ℃,恒温5 min消除热历史后降至常温,再以10 ℃/min的升温速率升温至200 ℃,记录升温曲线.
1.3.2 发泡样品的泡孔结构、压缩性能和电导率
将发泡样品置于液氮中浸泡4 h后脆断并喷金,采用上述SEM观察脆断面的泡孔结构. 利用Image-Pro Plus软件对SEM照片进行分析,获得各泡孔的直径,计算其平均值作为泡孔平均直径,并由下式计算泡孔密度N (cells/cm3).
N=(ncellA)3/2 | (1) |
式中,ncell为所分析的SEM照片上泡孔的个数,A为泡孔统计区域的真实面积(单位为 cm2).
采用电动拉压力试验机(ZQ-990B,东莞智取精密仪器有限公司),对微孔圆片进行多循环压缩/卸载测试,以残余应变(即每一次压缩/卸载后压缩应力回复为0时对应的应变[
将复合材料微孔圆片夹在两片铜箔之间,采用静电计(MT6514S,美国 Keithley)测量其体积电阻(R). 采用下式计算微孔圆片的电导率(σ):
σ=L/RS | (2) |
式中,L和S分别为微孔圆片的厚度和其与单片铜箔之间的接触面积. 测试5个微孔圆片,结果取平均值.
1.3.3 传感器的传感性能
将复合材料微孔圆片夹在两片铜箔之间,将其贴在聚酰亚胺(PI)基底膜上,并用聚乙烯(PE)薄膜作为保护膜,以封装成传感器(
Fig. 1 Schematics of assembled sensor in this work.
高分子复合材料中导电填料的分散和分布不仅影响其导电性能,还会影响其黏弹性,进而影响其发泡材料的泡孔结构等[
Fig. 2 SEM images of cryofractured surface of POE/MWCNTs/CF composite sample.
Fig. 3 DSC melting curves of POE and POE/MWCNTs/CF composite samples.
参考上述的DSC测试结果,设置3种饱和(发泡)温度(55、60和65 ℃),研究其对POE/MWCNTs/CF复合材料发泡样品泡孔结构的影响.
Fig. 4 SEM images of cryofractured surfaces and cell diameter distributions for foamed POE/MWCNTs/CF composite samples prepared at foaming temperatures of 55 ℃ (a), 60 ℃ (b), and 65 ℃ (c) (mean diameters and densities of cells and expansion ratios of foamed samples are given in right columns).
对上述3种发泡温度下制备的复合材料微孔圆片进行50次、50%应变的循环压缩/卸载测试,其中第1、10、20、30、40和50次循环的压缩应力-应变曲线如图
Fig. 5 (a)‒(c) Stress-strain curves of multiple cyclic compression/release tests, (d) residual strain versus cycle number curves, (e) compression stress-strain curves, and (f) compression strengths, compression moduli and conductivities for microcellular POE/MWCNTs/CF composite disks prepared at three foaming temperatures.
对复合材料微孔圆片进行50%应变的压缩测试,所得应力-应变曲线如
由上述3种发泡温度下制备的微孔圆片封装成传感器(Sensor-55、Sensor-60和Sensor-65;
Fig. 6 Relative resistance change (ΔR/R0) versus strain curves for three kinds of sensors.
3种传感器所呈现的灵敏度差异与其对应的微孔圆片的泡孔结构和泡孔壁上导电填料的分布密切相关. 下面结合
Fig. 7 Typical SEM images of cell walls and schematics of their deformation in microcellular POE/MWCNTs/CF composite disks for (a) Sensor-55, (b) Sensor-60 and (c) Sensor-65.
Sensor-55对应微孔圆片的泡孔壁较厚且连续性高(
Sensor-60尤其是Sensor-65对应微孔圆片的泡孔壁明显较薄(图
采用冷冻干燥法[
由上述结果可知,55 ℃发泡温度下制备的微孔圆片在压缩过程中呈现较高的回弹性,由其封装的传感器(Sensor-55)具有良好的线性响应范围和灵敏度. 对该传感器在不同的压缩速度和压缩应变下的电阻响应进行测试,所得R/R0随时间的变化曲线见图
Fig. 8 Electromechanical behaviors of Sensor-55. Resistance change (R/R0) under (a) four different compression speeds (25% strain), (b) step cyclic strain in five cycles and (c) five different compression strains in ten compression/release cycles (3 mm/min compression speed); (d) durability test for 1000 compression/release cycles (3 mm/min compression speed and 30% strain).
为发挥Sensor-55具有较宽线性响应范围、较高回弹性等优势,将其用于手指按压、肘部弯曲、深蹲和脚踩等典型人体运动的检测. 测试时,采用导线将Sensor-55接入静电计,记录其电阻随时间的变化,获得电阻变化(R/R0)随时间的变化曲线. 通过手指在Sensor-55上作用小压力(对应小压缩应变)并保持一段时间,其电阻快速减小并保持一较稳定的值,移开手指后,电阻快速回复至其初始值(见
Fig. 9 Monitoring of typical human motions by using Sensor-55. Resistance change (R/R0) versus time curves during (a) finger pressing, (b) elbow bending, (c) squatting and (d) foot stepping.
通过熔体混炼制备POE/MWCNTs/CF复合材料,采用Sc-CO2作为物理发泡剂在55、60和65 ℃ 3种发泡温度下对其进行釜压发泡. 55 ℃发泡样品的泡孔结构较为均匀,泡孔直径分布较窄(主要在10~30 μm内),泡孔壁较厚且连续性高,从而具有较高的回弹性、压缩强度(约630 kPa)、压缩模量(2.95 MPa)和电导率;60 ℃发泡样品泡孔的直径较小、密度较高,泡孔壁较薄;65 ℃发泡样品泡孔的直径明显较大、密度较低、直径分布明显较宽,泡孔壁很薄. 采用60和65 ℃下制备的微孔圆片封装的传感器在0~40%压缩应变范围内呈现2~3段GF值不同的线性区,GF总体上较低. 采用55 ℃下制备的微孔圆片封装的传感器在较宽的线性响应范围(0~30%应变)内具有较高的灵敏度(GF为1.67),在不同的压缩速度和压缩应变下均能呈现较快速的响应和恢复性能以及良好的重复性,在1000次(约8350 s)循环压缩/释放测试中呈现较稳定的压阻响应和良好的耐久性,且对不同的人体运动所产生的范围较宽的压力(对应较宽的压缩应变)具有较好的检测能力. 本文工作对采用超临界流体发泡法制备弹性体为基体的微孔导电复合材料应用于传感等领域提供了一定的指导.
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